The present invention refers to a screening apparatus for sorting and classifying fluid flows, and in particular to a bar screen, especially for sorting fiber suspensions, and being of the type having a plurality of parallel bar-shaped screen elements mounted on cross bars for forming a screen area and defining slotted screening perforations therebetween which extend essentially transversal to the direction of the fluid flow.
It is known to provide bar screens with triangular screen elements, with their top surface, which faces the incoming fiber suspension, being levelled or flat so as to form a plane screen area. Practice has shown however, that the provision of irregular screening surfaces is advantageous for improving the screening capacity and sorting quality because of the hydrodynamic effect of the screen and the generation of so-called microturbulences. It was thus proposed to slantingly position the screen elements so that their top surface is angled relative to the screen area. In this manner, an approximately stepped screening surface is created.
For manufacturing reasons, the angle of inclination of the screen elements is limited so that the depth of the thus non-levelled screening surface is also limited. Theoretically, the depth could be increased by expanding the width of the screen element, however, such measure would result in a loss of free surface area, thus causing a reduced throughput. Moreover, a slanted arrangement of the screen elements in order to attain an irregular screening area is disadvantageous because it results at the outlet side of the screen in an asymmetric flow space which adversely affects the fluid flow and causes pressure losses.